Field of the Invention
The present invention relates to an image forming apparatus having a charging member for charging a photosensitive member.
Description of the Related Art
In an electrophotographic or electrostatic recording image forming apparatus, a corona charging unit or other charging unit has conventionally been used to charge an image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric member.
In recent years, there has been put into practice a charging unit in which a voltage is applied to a charging member disposed in contact with or in the vicinity of a to-be-charged member (e.g., an image carrier) to thereby charge the to-be-charged member. This charging unit is advantageous in low ozone emission and low power consumption.
Such charging units are classified into two, one of which is a charging unit of DC charging system where only a direct voltage is applied to the to-be-charged member via the charging member to charge the to-be-charged member, and the other of which is a charging unit of AC charging system where an oscillatory voltage having an alternating voltage component and a direct voltage component and having a voltage value that periodically changes with time is applied to charge the to-be-charged member.
The charging unit of AC charging system is excellent in charging uniformity and widely used in recent years. However, the oscillatory voltage applied from the charging member to the to-be-charged member has a voltage value that changes between positive and negative values, so that discharge and reverse discharge are repeated between the charging member and the to-be-charged member. Due to the discharge from the charging member to the to-be-charged member, a surface of the to-be-charged member (e.g., a photosensitive drum) is deteriorated. This poses a problem that the deteriorated surface of the photosensitive drum is worn by friction with a contact member such as a cleaning blade.
To obviate this, many methods have been proposed to properly control the amount of discharge current in the charging unit of AC charging system. For example, an image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2007-033948 is configured as follows: The charging member is applied with an alternating voltage having a peak-to-peak voltage lower than a discharge starting voltage at which discharge is started between the charging member and the photosensitive member, and the peak-to-peak voltage of the alternating voltage is increased stepwise. A current flowing through the charging member when each of alternating voltages of peak-to-peak voltages is applied to the charging member is detected, and the detected current is decomposed into integer-order frequency components. Then, the peak-to-peak voltage where the fifth or higher order frequency component becomes a peak is set as a peak-to-peak voltage of the alternating voltage to be applied to the charging member for image formation.
However, with the method disclosed in Japanese Laid-open Patent Publication No. 2007-033948, electric potential on a surface of the photosensitive member changes while control is being performed based on this method since the peak-to-peak voltage of the alternating voltage applied to the charging member is changed stepwise from a voltage lower than the discharge starting voltage. When image formation is performed in a state where the electric potential on the surface of the photosensitive member changes, toner fogging or density unevenness is generated. This poses a problem that the alternating voltage applied to the charging member cannot be adjusted in a period in which a charging operation for image formation is being performed.